At the present, there are in the U.S. alone upwards of 105,000 inadequate bridges. A majority of them are functionally obsolete while a lesser number of them are structurally deficient. The latter are defined as bridges which had to be restricted to light vehicles only or closed, while the former are identified as bridges which can no longer safely service the system of which they are an integral part. The replacement cost for these bridges is in the tens of billions of dollars. A majority of these bridges are relatively short span bridges, say bridges having a length of 30 to about 100 feet. Applicants have recently invented a bridge system ideally suited for building such bridges with relatively low production and erection costs. Although this system is expected to greatly facilitate the replacement of these shorter bridges, it is relatively less well suited for incorporation in long span bridges, say bridges which have a clear span of 100 feet or more up to several hundred feet. Generally, such bridges are constructed as continuous, cantilivered, suspension, or arch bridges.
Whatever the particular construction of the bridge, the load or traffic carrying surface is intermittently supported over its length, either by piers or with suspension cables. The bridge deck and more specifically the support structure for the deck must have sufficient strength and rigidity to carry the load between the support points.
The probably most common manner of supporting the bridge deck between the above discussed support points is by providing suitable beams or girders which carry the deck. For relatively short spans (between support points) extruded steel profiles may suffice. For longer spans, however, it is necessary to fabricate structures to achieve the necessary strength and rigidity without requiring excessive amounts of materials. Here, one of the most common forms of construction is to provide a supporting steel framework, usually made up of plate, angle, channel, etc. which are welded or riveted together. For relatively long spans and/or for heavy loads an efficient support structure are so-called box beams which have a relatively high strength to weight ratio.
Conventional box beams are made of flat plates that are typically welded to each other. Inspite of their advantages over prior art forms of long span, high strength and rigidly fabricated support beams, they remain relatively heavy. Flat plate in many instances is an inefficient geometric configuration for carrying a variety of loads, particularly shear and bending loads. The latter and in particular, the shear stresses that must be carried by the box beam, which typically is several feet in height, may result in a buckling of the vertical beam wall unless it is supported at intermediate points over its height. According to the prior art, this is accomplished by securing, typically welding stiffeners which have substantial depths (perpendicular to the flat sheets of which the box beams are constructed) such as angle irons, channels and the like to either the inside, the outside, or both of the walls. Since at least the upper chord plate of the box beam is subjected to significant compression forces, which may again cause the buckling of the plate, it too must be stiffened in a manner analogous to that of the side walls of the beam.
The stiffening members attached to the flat walls of prior art box beams are normally welded thereto, frequently over their entire length to avoid the formation of pockets which may collect moisture and which may result in an accelerated corrosion of the underlying metal. The great deal of welding that is required is not only time consuming and, therefore, expensive, it normally results in locked in stresses or outright damage to the base metal adjacent the welds. Further, stresses due to strinkage when the weld metal cools may lead to hairline cracks which may not form until some time after the beam has been assembled and installed. Needless to say, such cracks are difficult and, therefore, expensive to detect and, more seriously, if they go undetected they pose a serious danger to life and property. At the very least, once detected they may require expensive corrective work in the field.
U.S. Pat. No. 3,181,187 is illustrative of a bridge construction which employs longitudinally extending box beams for supporting the bridge deck and road surface.